1. Field of the Invention
The field of the invention is that of the acquisition of seismic data. More precisely, the invention relates to equipment for analysing the sea bed. The invention relates in particular to the oil prospecting industry using the seismic method, but can apply to any field using a system for acquiring seismic data in a marine environment.
2. Background of the Invention
The operations of acquiring geophysical data on site conventionally use networks of sensors (designated by the term “hydrophones” with regard to the acquisition of data in a marine environment). The hydrophones mentioned previously are distributed along cables in order to form linear acoustic antennae normally referred to as “streamers” or “seismic streamers”. The network of seismic streamers is towed by a seismic boat. A seismic streamer consists of an assembly of individual sections instrumented with seismic sensors and associated analogue to digital conversion electronics. The seismic boat also tows one or more seismic sources consisting of a network of air cannons, water cannons or acoustic vibrators. The pressure wave generated by the seismic source passes through the column of water and insonifies the upper layers of the sea bed. Part of the signal is refracted by the interfaces and inhomogeneities of the ocean crust. The resulting acoustic signals are then detected by the seismic sensors distributed over the entire length of the seismic streamers. These acoustic signals are processed, digitized and retransmitted by the telemetry from the seismic streamers to the operator station situated on the seismic boat, where the processing of the raw data is carried out.
To have a correct image of the mapping of the sea bed in the exploration zone, it is important to precisely locate the seismic sensors distributed along the seismic streamers as well as the seismic source. This location of the streamers is important, in particular in order to:                follow the position of the hydrophones in order to obtain satisfactory precision in the image of the sea depths;        detect the movements of the streamers in relation to each other;        follow the navigation of the streamers, in particular in a situation of passing round an obstacle such as an oil barge.        
It should be noted that seismic streamers consist of an assembly of sections generally having a length of approximately 150 meters, the streamers being able to have a total length of several kilometers (6 to 7 kilometers, but possibly ranging up to 10 or even 12 kilometers).
Various methods are used for precisely locating the position of the seismic sensors distributed along the streamers. Initially, the positioning of the marine seismic streamers and seismic source have been based on the use of GPS receivers, distributed at a few particular points (boat, buoys supporting seismic source, buoys at the head and tail of the streamers, etc), and magnetic compasses distributed in larger numbers along the streamers (these compasses making it possible to determine deformations in the streamers). Currently the techniques for positioning marine seismic streamers are more efficient. They still use GPS positioning in order to have the absolute geographical reference frame, but they associate with this the use of submarine acoustics in order to determine the distances between acoustic modules mounted along the seismic streamers. These acoustic modules may be transmitters and/or receivers and make it possible to determine the distances between the adjoining modules situated on the various streamers. There are then available, in order to achieve precise positioning of all the streamers, reference points given by the GPS receivers and a mesh of inter-module and therefore inter-streamer distances.